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A puzzler!

I agree, that even if one gets an apparent boost to lift, the corrolary laws of thermodynamics say that "you don't get anything for free." However, it may be that even if there is a cost penalty (i.e., source of air) to pressurize the inside of the wing, maybe that penalty is smaller than the relative gain to be had in lift. In other words, maybe it is more efficient in generating lift to, say, pressurize the wing and add microholes on the upper surface than it is to change the shape of the wing, or add more power or whatever.

One would think that all this sort of stuff had already been considered by the people whose job is to design wings, but maybe not....

cheers,
greg
 
I just looked up the specs on the 777-300ER, with a 212.6' wingspan and a maximum landing weight of 554,000 lb. My retired airline buddy says that his DC-10 used a final approach speed of 135 kt, so using this speed and 500,000 lb the average downwash velocity would be about 26 fps, 17 mph.
 
I just looked up the specs on the 777-300ER, with a 212.6' wingspan and a maximum landing weight of 554,000 lb. My retired airline buddy says that his DC-10 used a final approach speed of 135 kt, so using this speed and 500,000 lb the average downwash velocity would be about 26 fps, 17 mph.

Which would dissipate itself rather quickly and would be difficult to notice, unless conditions were just right and you were looking for it.
 
...first and second law of thermo (paraphrased "You can't win and you can't break even"). Smarter minds than mine have already thought about this for many years. Fun thread though.

I love that paraphrase! :)


One would think that all this sort of stuff had already been considered by the people whose job is to design wings, but maybe not....

cheers,
greg

Yep...imagine a forum for the Wrights, Curtis', Voughts, McDonnells and Goddards of the 20th century. I can imagine it, but I doubt I would have been invited in! :eek:) Imagine what they could have accomplished with a computer! (But the world has such minds in it now too, with computers...if we could only get them from Facebook to Aerodynamics!...OK, I'm kiddin'...sort of!)

I just looked up the specs on the 777-300ER, with a 212.6' wingspan and a maximum landing weight of 554,000 lb. My retired airline buddy says that his DC-10 used a final approach speed of 135 kt, so using this speed and 500,000 lb the average downwash velocity would be about 26 fps, 17 mph.

Which would dissipate itself rather quickly and would be difficult to notice, unless conditions were just right and you were looking for it.

Those 747s at LAX and MSP would have had to have been really low on the slope for me to feel it. Hmmmm....wonder if we stood on top of the Laurel Street Garage on short final to San Diego Lindbergh (SAN)...:rolleyes:

Cheers,
Bob
 
Pressure in the wing, or lack thereof, has no effect on lift. Earlier in this thread someone brought up the example of a sqaure foot of lower wing skin, then discussed the pressures acting on the upper and lower surface of the one square foot piece. If you are going to conceptualize that one square foot and the forces on it, you also must consider all of the shear and moment loads on the edges of that sheet that are internal in the aluminum. Then the effects on the adjacent sheet. Etc. The net effect, if you add up all the sheets will be a net zero lift. If you make the squares small enough, in fact infinitesimally small, as the size approaches zero, and you continue to sum all of them up, it is, as you know, called integration. If you integrate around the shape of the wing, the result will be zero. The pressure on the outside is what provides the lift. Specifically it is the pressure on the lower side of the wing, and the fact that it is higher than the pressure on the upper side of the wing, that provides the lift. As I said before, it can be dynamic or static pressure, it doesn't matter, but the difference between the two causes lift. The pressure inside the wing, regardless of the shape of the wing, or wether it is hollow or solid, or if it has fuel in it or not, does not effect lift.

That said, you may be able to do something creative with air flowing (leaking) out of the wing in specific flight regimes, like stall, or cruise, or something, but it is the flow of air as it goes external to the airplane that will effect the lift, not the internal flow or pressure.

Tim.
 
In other words, maybe it is more efficient in generating lift to, say, pressurize the wing and add microholes on the upper surface than it is to change the shape of the wing, or add more power or whatever.

One would think that all this sort of stuff had already been considered by the people whose job is to design wings, but maybe not....

cheers,
greg

The X-21 project was a laminar flow wing project that used nearly a million slots to eliminate the boundry layer air. They found out that it worked as advertised, and was completely impractical as they couldn't keep the slots clean.
http://en.wikipedia.org/wiki/Northrop_X-21
http://books.google.com/books?id=C-MDAAAAMBAJ&pg=PA12&lpg=PA12&dq=boundary+layer+wing+holes+keep+clean&source=bl&ots=m2vCdTUNWK&sig=p5mEGusvk7cm4h6mm3Lt2P_bv7k&hl=en&ei=OuS0TMCiC8OC8ga3hd3_Dw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBwQ6AEwAQ#v=onepage&q=boundary%20layer%20wing%20holes%20keep%20clean&f=false
 
Rubber strips and canopies

As I mentioned before, the reason that those rubber sealings strips between the wing and fuselage come out unless RTVed in place is because the air inside PUSHES them out; maybe that wouldn't be a problem if they could be installed with the lip to the inside rather than outside, as with the baffles in the cowling, so that the pressure would hold them in place.
Now about forward opening canopies. The canopy has well over a hundred pounds of pressure differential (not psf) between the cockpit and the air outside of it in cruise, and lots more when pulling Gs. I figure that on my canopy the lift is at least 200 pounds, if not more. So if the latch lets go, the air inside the cockpit pushes it open. But how can that be if the air isn't pushing up on it? So maybe the air inside gets it started up, but then the outside air rushes in around the openings and that's what holds it open. 'That right? So then the air inside is no longer holding it up, so that satisfies the idea that the air inside can't sustain lift, only outside air. 'That it?
 
I see where you're going with it Paul, but I still think its the differential pressures on the outside of the structure of the wing that results in lift. As Tim mentioned, the pressure internal to the wing is acting on all inner surfaces of the wing. I'd say the same thing is true in the cockpit.

However, the canopy is a lifting body due to its shape (the canopy of the F-14 was the point at which critical mach was reached first, so air definitely accelerates over the curve of a canopy)...so there is lift being provided by the canopy. But with the canopy closed, the differential causing the lift is the diff pressure between the top of the fuselage/canopy, and the bottom of the fuse (because the canopy is secured down, and thus part of the overall airframe).

Once you open the tip up canopy however, it becomes a lifting body sort of on its own, and the pressure differential is between the upper and lower surface of the canopy...it lifts because of the acceleration of air over the top of it (except at the hinge, where its still connected to the airframe).

That's perhaps oversimplified, and certainly just my opinion. Methinks the only way to prove or disprove your theory that pressure inside the wing adds to the lift (I think that's the theory ;)) would be to completely seal a leaky wing, and compare the before and after...methinks! :)

Thoughts?

Cheers,
Bob
 
Paul, the air is pushing just as hard on the floor of the cockpit as it is on the bottom of the canopy. When you add up all of the air pushing on internal parts (using vector math), the resulting force is zero (or really close since there is some leaking air).
 
Steel balloon

'Don't know if you're aware of this, but the Atlas ICBM and later Space Vehicle was a steel balloon. It was made of about 0.010" SS inthe upper regions, and about 0.015" SS down around the engines. It used pressure inside to inflate it and make it stiff without any airframe. Ma Nature relies on that same principal, in many cases,to achieve procreation.
There was a lot of conjecture about whether this structure-less approach would hold up under the aerodynamic loading, and it was proved feasible when in an early test launch the missile did an outside loop shortly after lift-off and held together. When it wasn't pressurized, as on the transporter-erector trailer, it was held in stretch so that the thin SS sheet wouldn't fold and crease.
I'm not really saying that this "air inside the wing giving lift" theory, which isn't really a theory but a hypothesis, is truly how lift is occuring, but as a hypothesis it is as credible as any other of the several hypotheses that abound on how lift takes place. We have Bernoulli, horseshoe circulation, and several other novel ideas competing, and as yet I haven't heard where there is concensus. It doesn't mean that the airfoil and wing planform data can't be used to achieve a successful design, it's just that, as far as I know, no one lift expanation has been ruled and proven to be the one and only.
I only advance this as I see no works, other than one, that mention that the air inside the wing must be taken into account in coming up with the structure and the forces at work.
So again, I'll ask, taking into account the summation of vector forces and such, is all of the lift only on the lower surface and what part does the upper surface play in all of this?
 
Thought experiment

Well, nobody seems to have picked up on my post about the pressurized fuselage, so I'll try this thought experiment.

Consider a wing with a symmetrical airfoil section. For the sake of this discussion, consider it to be in a wind tunnel being tested. Consider the wing to be a closed structure and sturdily built to withstand internal pressure. Initial conditions are zero airspeed and zero angle of attack.

Now we connect the inside of the wing to an air pressure source and pressurize the inside of the wing to 50 psi. Vector sum of the pressure acting on all of the internal wing surfaces is zero although there is a structural load on the wing skins and ribs to resist the internal pressure. Now start up the wind tunnel and adjust the airspeed to 100 mph. Zero angle of attack, zero aerodynamic lift. Zero lift from the internal pressure. But there is an aerodynamic drag force caused by the airflow. Now vent the internal pressure to atmosphere (doesn't matter if this is vented to the tunnel air or to the outside air). Zero aerodynamic lift, zero lift from the internal pressure but still have the aerodynamic drag force.

Now increase the angle of attack to +10 degrees. Even though the wing section is geometrically symmetrical, the relative wind moved the stagnation point down around the leading edge and as far as the wind is concerned, the airfoil no longer looks symmetrical. The air moving under the wing has its momentum changed by the wing and deflects downward, at the same time imparting a slightly higher than freestream pressure on the lower wing surface. The air above the wing also has its momentum changed by trying to follow the wing surface (we're not yet at the stall angle) which results in a slightly lower than freestream pressure on the upper wing surface. The vector sum of all the pressures acting on the upper and lower wing surfaces is now upwards (aerodynamic lift), and we still have a drag component. Now we repressurize the inside of the wing to 50 psi. There is no change in the aerodynamic lift or drag forces.

So how does a wing create lift? It is an aerodynamic effect of placing a surface in a moving fluid stream, with emphasis on "dynamic". If you place manometer tubes over the top and bottom surfaces of the wing you will be able to visualize the higher pressure on the bottom and lower pressure on the top of the wing. And if you vector sum these you will see that there is a net force created that we call lift.

If you turn on the smoke streams in the wind tunnel you will also see the streamlines get closer together on the bottom (higher pressure) and farther apart on the top (lower pressure). This is the Bernoulli explanation of lift. But you will also notice something else. The streamlines leave the wing at a different angle than the freestream ahead of the wing, that is the wing has imparted downward velocity to the air. This is the Newtonian explanation of lift, that is F=MA. Both explanations are valid in showing how a wing creates aerodynamic lift force. I prefer the Newtonian explanation as I think it gives a better explanation of the dynamic aspect of how a wing can lift the weight of an airplane.

Now getting back to the origins of this thread. Few wings have the interior sealed in the way our thought experiment wing was (and fewer still would withstand 50 psi internal pressure!). So because of drain holes, slots, gaps etc. a real wing is vented to the atmosphere. Depending on where the venting is relative to the pressures on the outside of the wing, it may have slightly higher or lower pressure than the freestream pressure. However, the forces that we have discussed are caused by aerodynamic effects, that is the movement of air over the surface (wind tunnel) or movement of the surface through air (airplane). How you think about this depends on your frame of reference. Now inside the wing is it possible to have different pressures on the inside of the upper wing skin and the lower wing skin? Only if there is an aerodynamic effect (air movement) inside the wing. If the wing leaks air badly enough that there is an internal aerodynamic effect, you might be able to measure a pressure differential between the inside of the top skin and the inside of the bottom skin. But you need to make lots of measurements to make sure this effect exists over all surfaces inside the wing and is not just a local effect. In any case I would suggest you could improve the efficiency of the wing by plugging the holes and relying on the aerodynamic effects on the outer wing surface to produce your lift.

OK, flame suit is on and zippered.
 
Well said, Terry!

And to elaborate on your final point ("In any case I would suggest you could improve the efficiency of the wing by plugging the holes and relying on the aerodynamic effects on the outer wing surface to produce your lift."): In a leaky hollow wing like that of an RV, there will certainly be some air movement inside, and it's unlikely to be doing anything constructive. The air rushes in at a leak point where external pressure is relatively high and exists at another leak point where external pressure is lower, working its way there through an obstacle course of internal structure (ribs, spar) along the way. By far the dominant aerodynamic contribution of this flow will be drag. And that's why, as Terry suggested, it would be advisable to minimize the airflow through the inside of the wing structure.
 
Terry and Roee,

Both really well stated posts. Terry, you summed up what a lot of us have been saying in a more obtuse way (well, at least mine were obtuse! :rolleyes:). You also made sense (for me) of the Newtonian aspect, and it jives with our experience of little downwash (relative to a rotorcraft). Paul's post on the math behind it made sense, but you put it in plain language...thanks! And I think Newton and Bernoulli can live in harmony!!

And Roee, you brought it to its practical terms, and I'll follow that with the next logical (to my illogical mind ;)) step. Can we plug the holes and stop the leaks and the drag? It goes back to my quip, "I wonder if I have enough tubes of RTV to do that", and I still ask...could you do this and not increase weight to the point that the thang won't fly?

Perhaps that Wright/Curtis/McDonnell/Goddard forum had a thread on this, and they accepted some leakage as a compromise to weight savings and flyability...the early version may have said, "man, if we could only make the wings from metal, and ditch this fabric stuff, we could really lose the leaks"! :p I, for one, am glad they stuck with fabric for a long time (still like fabric!!) :D

Cheers,
Bob
 
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many gliders use blow holes

many gliders use blow holes to control transition and laminar separation.
They don't leave it up to chance what the wing internal pressure might be though, they have little pitot probes that pressurize a plenum in the wing that supplies air to the blow holes.



The air inside the wing, whatever its pressure might be, pushes down just as hard on the bottom skin as it pushes up on the top skin. It therefore has a zero net effect on lift. Sorry to spoil the fun :)

Your observation about the higher pressure inside the wing does raise an interesting thought though. Is there a way that we really could use it to our aerodynamic advantage?

One idea that comes to mind is to port it to points along the top surface of the wing where it can be used to accelerate the flow with sort of a "jet" effect (but not enough of it to significantly raise the pressure above the wing and therefore spoil the lift). Know what I mean? Serve a purpose similar to vortex generators, to accelerate the flow to help keep it attached at high alpha, but do it by means of small jets of high pressure air. In its simplest form, you could just drill tiny holes through the top skin at a very shallow angle facing aft. And unlike VG's, you could deploy these selectively during flight, only at high alpha when they're needed, by simply opening or closing the ports from the inside of the wing. Now that I think about it, the idea is pretty similar to a slat. But much lighter and simpler mechanically. Hmm...
 
Even with all of these wonderful explanations, no one has yet written about whether all of the lift is on the bottom surface. How does the lift actually manifest itself on the wing. We do know that suction is not a force so that there is nothing pulling up on the top surface. So if it's all on the bottom, then there is no force acting upward on the top skin other than the inside-outside differential which must be accounted for in engineering the attachment of the ribs to the skin and to the spar. The upper surface is only used to generate a lower pressure than on the bottom so that all of the upward force is on the bottom. Is that how it is?
BTW, if you have Abbott & VonDoenhoff's book on wing section theory, take a look at the pressure distributions on pages 334-381 of the laminar flow sections at zero and at an increased positive alpha and you'll see that the airfoil has low pressure on both the top and the bottom at zero alpha, then as alpha goes up, the top's low pressure drops even more, and the bottom's low pressure decreases.
As an example, the 661-015 low pressure top and bottom peaks at 1.4 at 60+% chord, then at a CL of 0.2, the top flattens out at 1.45 from 0 to 60% chord, and the bottom peaks at about 1.35 at 60%.
Another BTW. Can anyone explain why a wing at high CL will generate more lift than what is present in the dynamic pressure? If lift equals Q times CL times area, how is it that without flaps we can get 1.4 to 1.6 times as much lift as Q has, and with slotted flaps and LE devices it can be 2.6 times Q, as shown on P.229, ibid. Wings can multiply dynamic pressure? WOW! That's magical!
 
Even with all of these wonderful explanations, no one has yet written about whether all of the lift is on the bottom surface. How does the lift actually manifest itself on the wing. We do know that suction is not a force so that there is nothing pulling up on the top surface. So if it's all on the bottom, then there is no force acting upward on the top skin other than the inside-outside differential which must be accounted for in engineering the attachment of the ribs to the skin and to the spar. The upper surface is only used to generate a lower pressure than on the bottom so that all of the upward force is on the bottom. Is that how it is?

sorry don't have the abbot &dornhoff book so i deleted that part.

suction might not be a force, but i think that it is something that laymen would use to describe pressure differential.

re: "is that how it is?" kind of sounds like how it is to me. whether it is lower pressure due to faster air movement or higher pressure on the lower surface due to higher cross surface exposed to prevailing wind, or a combo of the two; thats what i think keeps a wing flying
 
Even with all of these wonderful explanations, no one has yet written about whether all of the lift is on the bottom surface. How does the lift actually manifest itself on the wing. We do know that suction is not a force so that there is nothing pulling up on the top surface. So if it's all on the bottom, then there is no force acting upward on the top skin other than the inside-outside differential which must be accounted for in engineering the attachment of the ribs to the skin and to the spar. The upper surface is only used to generate a lower pressure than on the bottom so that all of the upward force is on the bottom. Is that how it is?
BTW, if you have Abbott & VonDoenhoff's book on wing section theory, take a look at the pressure distributions on pages 334-381 of the laminar flow sections at zero and at an increased positive alpha and you'll see that the airfoil has low pressure on both the top and the bottom at zero alpha, then as alpha goes up, the top's low pressure drops even more, and the bottom's low pressure decreases.
As an example, the 661-015 low pressure top and bottom peaks at 1.4 at 60+% chord, then at a CL of 0.2, the top flattens out at 1.45 from 0 to 60% chord, and the bottom peaks at about 1.35 at 60%.
Another BTW. Can anyone explain why a wing at high CL will generate more lift than what is present in the dynamic pressure? If lift equals Q times CL times area, how is it that without flaps we can get 1.4 to 1.6 times as much lift as Q has, and with slotted flaps and LE devices it can be 2.6 times Q, as shown on P.229, ibid. Wings can multiply dynamic pressure? WOW! That's magical!

Paul, you might be "overthinking" the issue. Flaps change the camber of the wing, which add downward acceleration to the mass of air flowing over the wing. F=ma. When a is increased, the lift increases. The pressure difference between the top and bottom of the wing is what accelerates the mass of air flowing over the wing downward. Also causes the wing tip vortices.

As you go faster, say cruise airspeed, you effectively increase the mass of air flowing over the wing, so you need less "a", as mentioned in a previous post.

At cruise airspeed, if you pull back real hard on the stick you increase the angle of attack of the wing, which makes "a" larger in the F=ma equation. the result is more lift, which the pilot feels as "G's".

Then there is inverted flight. Same equation, same forces, same result.

Easy. :)
 
Pressure in a sealed enclosure acts normal (perpendicular) to the surface. Thus even a wing shaped enclosure with more surface area on the upper half than the lower half does not lift off of the ground when pressurized. The sum of the forces is zero. This is still just an application of F=ma, where a = 0.

A simple thought experiment would be to devise a shape for the air compressor tank that could lift itself off the ground simply by filling it up.

Now if we do have moving air, we can still ignore the internal area and sum up the inward and outward flows, which in a steady-state must be equal.

If we look at the smoke trails of a wing in a wind tunnel, we do see those trails on top of the wing turned downward, and after the wing they are lower than when they began. We also put spoilers on top of the wing, and I hear they're very effective at destroying lift.
 
Top vs. bottom surface "lift"

Even with all of these wonderful explanations, no one has yet written about whether all of the lift is on the bottom surface...

I'll try to answer your question, but I think there's a bit of a semantic "gotcha" in how you're describing the problem. Let's try to avoid confusion and boil down the problem to its simplest form. Bare with me.

We'll describe a hollow wing section that is fully enclosed in a rigid skin. We'll say it's an infinite span, so we're not confounding the problem with edge effects. The internal space is at some arbitrary but uniform pressure (no flow).

Now, we put this wing section in the airstream, and at a given set of parameters (air density, speed, angle of attack...) we get a distribution of external air pressures all around the profile of the wing.

Knowing that distribution of external air pressures, and the internal pressure, we can calculate the distribution of force all the way around the profile of the wing. Let's talk in terms of very small but finite elements, since that tends to be easier for people to visualize. At each of these elements, the force vector is the difference between the inside and outside pressure divided by the surface area, acting perpendicular to the plane of the surface.

From this, we can sum up all the force vectors all the way around the profile, and what we get is the lift vector of the wing (and we can also look at the moments and determine the center of lift, pitching moment, but I digress...).

Notice that this is the first time I've mentioned the word "lift". I deliberately didn't want to describe the individual finite element force vectors as "lift", because that really starts to lose meaning.

For example, if I arbitrarily make the pressure inside the wing really really high, let's say a thousands times greater than the external pressures, that'll make all those little force vectors really really big. The force will be real, and it'll certainly push out on the skins. But it doesn't make much sense to think of all that force as "lift". It certainly isn't aerodynamic lift. It's a static force, only one small component of which -- the component caused by the external pressure -- that's actually created by aerodynamic effects.

It is also important to note, again, that because the pressure inside the wing acts uniformly on the skin all the way around, its contribution to the force vectors all the way around sums to zero. In other words, it has no net effect on lift. Whether the internal pressure is zero psi or a million psi, the skins will experience different forces, but the overall lift vector comes out the same.

What does change with internal pressure, is the force distribution along the skins. Yes. If your interest is what proportion of lift is supported by the top skin versus the bottom skin, then yes, you can manipulate this proportion by manipulating the internal pressure. But I don't think it makes much sense at that point to describe those forces individually as "lift". That's the gotcha. So humor me and let's just talk in terms of "force".

Disclaimer: Now I'll gloss over many details for the sake of discussion, but not affecting the results...

If you set the internal pressure to be the same as the external pressure over the top of the wing, then the bottom skin will experience 100% of the upwards force and the top skin will experience no net force.

If you set the internal pressure to be the same as the external pressure under the bottom of the wing, then the top skin will experience 100% of the upwards force and the bottom skin will experience no net force.

By manipulating the internal pressure you can achieve any proportion in-between that you like.

And by manipulating the internal pressure you can even achieve a situation where the top skin experiences more than 100% of the upwards force (100% referring to the total lift force), while the bottom skin experiences a downwards force counteracting the overage experienced by the top skin. And this is actually exactly what you get with the original situation that you described, where the internal pressure is greater than the external pressure both above and below the wing.

But ultimately I think the bottom line is this. The internal pressure can have implications with respect to stresses on the structure, and we can see the effect of this in the slight pillowing of our wing skins. But if we consider our wing to be rigid and ignore the stress and deformation issues as negligible, then what we're left with is that the pressure inside the wing, and the related notion of forces on the top versus bottom skin, are aerodynamically inconsequential.
 
Pillowing

My first thought on the pillowing seen on the upper skin is buckling of the skin due to the upper surface being in compression as the wing bends when creating lift. I think the deformation due to pressure differential would be small but it would add to the effect from buckling.
I may be wrong but had to put my 2 cents in.:)
Great discussion!!
 
Is sucking a push or a pull?

Warning: Semantics at play in the following boring, dry post.

Assumption: the term "suction" used in this context denotes a pressure differential, or delta-P. For the purpose of the remainder of this post, "suction" = "delta-P".

Is delta-P a 'force'? No. But it will RESULT in a force when applied. One need only look at the units of the thing in question (dimensional analysis, a very basic and useful thing).

Example units of FORCE -> lbf, (kilo)Newton
Example units of delta-P -> psi, (kilo)Pascal

Clearly different units. So how does delta-P ('suction', as defined above) result in a force?

Let's consider a simple arbitrary example. You and a friend are moving a 4'x8' sheet of plywood, when a gust of wind strikes 90 degrees to one of the flat surfaces, applying a 1psi delta-P across the sheet. That delta-P results in a loading of [(4ftx12in/ft)x(8ftx12in/ft)] x 1lbf/in^2 = 4608 lbf (That was a big gust).

Now here are the semantics. Did the pressure differential push the sheet of plywood from the upwind side, or pull it from the downwind? Don't waste time arguing the point, its just semantics. Both are wrong, and both are right. Doesn't matter. What counts at the most basic level is that there was a delta-P, with the resultant acting towards to lower pressure (hi to low, look out below).
 
O.K., if it is not suction, and it is not reaction, and it is not Bernouli, and it is not Conda, and probably not even Einstein, well then the only thing left is......


MAGIC!!:p
 
So the pressure differential over the top skin in level flight is about .27 psi. What about during an accelerated stall? I would assume internal pressure remains the same, but there would be a rapid drop over the external upper surface. Could this not cause failure in the upper rivets? I never thought of this issue before this discussion. Interesting stuff.
 
I see that as yet everyone seems to be skating around the question of "is all of the lift, then, on the bottom surface." Somewhere on the wing there must be an upward force acting on it to generate what we term lift. What part of the wing structure is that? Bottom, top, magic?
And if lift is Q X A X CL, and CL is 2.6, how does the wing magically multiply the dynamic pressure Q so that the "lift", or supporting force, can be greater than the applied force? Inquiring minds want to know! :confused:
 
I see that as yet everyone seems to be skating around the question of "is all of the lift, then, on the bottom surface."

The answer is simple. No. The actual physical force lifting the aircraft is the sum of the pressure differences across all of the surfaces of the aircraft. Every inch of every surface on the airplane is pushing the body in one direction.

I think you're confusing yourself by comparing the actual forces acting on the a/c and all of the models (circulation, etc) used to simplify those forces into something much easier to work with.
 
Dynamic Pressure

Q is dynamic pressure defined as Q=1/2*rho*V^2 where rho is the density of air and V is the freestream velocity. Its units are LB/FT^2. It is a measure of the pressure that would result on a surface if all of the kinetic energy were converted to pressure energy. CL, coefficient of lift, is a dimensionless coefficient of an airfoil's ability to convert dynamic pressure into a lift force (airfoils also have drag CD and pitching moment CM coefficients). A is the area of the wing on which the pressure is acting. Thus the equation Lift=CL*Q*A.

But an airfoil does not convert the kinetic energy of the air into pressure energy except along a line on the leading edge at the stagnation point. An airfoil redirects the air in such a way that it produces lift (with an attendant drag)

I think of CL as a measure of the airfoil's efficiency to produce lift by creating a delta-P between the top and bottom surface and to accelerate the air downwards and in so doing incur the least amount of flow separation and viscous drag. So the higher the CL the more efficient the airfoil is at producing lift.

As to which surface of the airfoil produces lift, it is both the top and bottom outside surface. You stated that "suction is not a force", well suction as a previous poster has stated is delta-P. The key is defining the baseline of measurement from which the delta is derived, in this case it is the static pressure of the ambient air.

Lastly, on the subject of internal air inside a wing, there are some airfoils (wings) that are completely solid (no internal air, therefore no internal air pressure), for example foam filled composite wings on a Rutan Long Eze, most propeller blades and most helicopter rotor blades.
 
The solid wing explanation is specious at best since the original postulate dealt with the action, or not, of the air within the wing. 'Sorta reminds me of the politcal arguments taking place now where one of them says, in reply " Yeah, but what about..."
The same for the "Yeah, but the air pushes down on the bottom surface as well as the top" or "But what if we put 5 tera pounds of pressure inside the wing? Then what?"
The whole proposition is that the air, inside the wing, is pushing up on the top surface with 40 psf, and pushing down on the bottom surface with 20 psf. Guess what? The top wins, with 20 psf more in the up direction.
I think what it boils down to is that basically you cannot bring your minds to accept whether air, that's carried along in the wing, can act as if it is totally independent and generate a force just as if it was outside the airplane. Do whatever you want with the amount of pressure in the wing. No matter what value you come up with, the upward force always wins!
So the mathematician sums up all of the forces acting all over the plane, and concludes that they show that these forces all act on the plane in an upward direction, but the engineer then says thats all great, but where on the structure do these forces act so I can engineer the strength that's necessary.
I showed that you had to accept that it is the air inside the cockpit that pushes up on the canopy when the latches fail, but I am met with idea that "Yeah, but the air is pushing down on the bottom of the cockpit, too." So what? So when the canopy opens, and outside air rushes in to hold it open, does that outside air not push down on the bottom of the plane, too? And yet it is the air pushing up on the canopy until the forces on both sides arrive at some position. I'd be willing to bet that when the canopy opened up that the plane had to assume a slightly higher AOA to make up for the loss of lift.
 
Closed System vs. Open System

I think you just disproved your original hypothesis. With the canopy example, when the canopy is latched the air inside the cockpit acts within a closed system, as does air inside the wing. When you release the canopy latch the delta-P acting on the canopy inner surface and outer surface opens the canopy to the point where the pressures and flows come back into equilibrium. In the case of the canopy locked, the air pressure in the closed system acts on the cockpit floor as well as the inside of the canopy. When the canopy is opened the pressures are unbalanced until a new equilibrium is established. If it was a design requirement, I think it would be possible to design a canopy so that there was no delta-P across it so that it wouldn't matter if the canopy was latched or not. It might have an odd shape and be difficult to see out of though.
Similarly with the wing example. The normal wing is a closed system. If a wing surface becomes detatched, for example a fabric covered wing surface tears away from the structure which has happened to some unfortunate pilots, any internal pressure is relieved and the system establishes a new equilibrium between pressure and flow.
I think the most important concept to remember in this discussion is the difference between the aerostatic and aerodynamic systems.
 
This has been interesting, way beyond my brain power, but interesting none the least.
if its air pushing up on the inside of the top wing surface that makes everything work, and solid wing design ideas are pretty much said to be invalid.........how do those neat little solid foam wings on toy airplanes fly? How do they create lift?
Sorry for the simple question, but, simple minds need simple answers.

Gary
 
This has been interesting, way beyond my brain power, but interesting none the least.
if its air pushing up on the inside of the top wing surface that makes everything work, and solid wing design ideas are pretty much said to be invalid.........how do those neat little solid foam wings on toy airplanes fly? How do they create lift?
Sorry for the simple question, but, simple minds need simple answers.

Gary

Magic....;)
 
The solid wing explanation is specious at best since the original postulate dealt with the action, or not, of the air within the wing. 'Sorta reminds me of the politcal arguments taking place now where one of them says, in reply " Yeah, but what about..."
The same for the "Yeah, but the air pushes down on the bottom surface as well as the top" or "But what if we put 5 tera pounds of pressure inside the wing? Then what?"
The whole proposition is that the air, inside the wing, is pushing up on the top surface with 40 psf, and pushing down on the bottom surface with 20 psf. Guess what? The top wins, with 20 psf more in the up direction.

I think what it boils down to is that basically you cannot bring your minds to accept whether air, that's carried along in the wing, can act as if it is totally independent and generate a force just as if it was outside the airplane. [...]

This is getting silly...

The static air inside the wing generates force against the skins. I think everybody here gets that. But the sum of the force vectors it generates along the top and bottom skins in total is always zero, so it makes no difference to the total lift produced by the wing. I'm not sure you get that. You can't gloss over this detail!

You could remove all the air from inside the wing, fill it with foam, fill it with lead, fill it with candy, or leave a vacuum -- it doesn't matter. The wing would still continue to generate just as much lift.

The air inside the wing does not push out on the top skin with greater force than it pushes out on the bottom skin. It pushes out on them with equal force.

The outside air on the other hand pushes in on the bottom skin with greater force than it pushes in on the top skin. These forces are not equal, and it is this difference between these forces that results in a non-zero net force upwards: lift. You don't seem to accept that.

Physical and mathematical explanations have been given. Examples have been given (the "what if"s) of cases that clearly demonstrate how your hypothesis fails. These are not specious. Demonstrating examples where a hypothesis fails is a perfectly valid means of disproving it. You refuse to accept these, and yet you haven't refuted them. You simply dismissed them off hand.

I'm done...
 
The outside air on the other hand pushes in on the bottom skin with greater force than it pushes in on the top skin. These forces are not equal, and it is this difference between these forces that results in a non-zero net force upwards: lift....

So, then, can I put you down as being in the camp that holds that all of the lift takes place solely on the bottom skin of the wing , i.e., that the upward pressure of the air is only felt on the wing's bottom surface? That being true, the air must also be pushing down on the earth surface with an equal force, just as the air inside the wing pushes up on the upper surface and down on the lower surface.
Another thing for lucubration: air has neither pressure nor temperature. It only possesses molecules that have a mean free velocity of about M1.4, V= (3*P/rho)^1/2, and what we call pressure and temperature, two sides of the same coin, are only a result of the collision of these molecules with whatever gets in their way.
 
To keep it simple and go back to your original question of, "So, could it be said that the wing's lift is really a result of the air in the wing pushing up on the top surface 20 psf harder than it is on the bottom? What say you?", the answer is no. The air in the wing does not push harder on the top surface than the bottom. It pushes just as hard up on the top surface as it does down on the bottom, so, no, the wing's lift is not due to the inside air pushing harder on one than the other. 'Tis impossible.

Your second question of, is the lift solely the result of the outside air pushing up on the bottom skin of the wing? The answer is no, lift is the result of the differential of the pressure, or when considered over an area, the differential of force pushing up on the bottom vs. that pushing down on the top.

Now the answer to your final question of, "The air must also be pushing down on the earth surface with an equal force?" is likely yes, but it is spread over such a huge area and and interfered with by so many other air currents that it is not only not noticed, it is not measureable.

Tim
 
So, then, can I put you down as being in the camp that holds that all of the lift takes place solely on the bottom skin of the wing...

No.

I think you're still confusing "lift" with the actual forces acting on various parts of the wing. "Lift" is a macroscopic concept. It is just a convenient construct for describing the vertical component of the net effect of all aerodynamic forces acting on a body. Sort of like "pressure" and "temperature" are convenient constructs for describing the macroscopic net effect of the motion and collisions of a whole bunch of individual molecules. In the context of lift, the top and bottom wing skins are not independent bodies. It therefore doesn't make sense to talk about the lift of the bottom skin versus the lift of the top skin, any more than it makes sense to talk about about the temperature or pressure of a single molecule. The macroscopic concept just doesn't apply at that microscopic level.

I'm sorry, I really don't know how to explain it any more clearly than the explanations already given here by myself and others. I think this discussion is going in circles at this point, so I'm out.
 
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No.

I think you're still confusing "lift" with the actual forces acting on various parts of the wing. "Lift" is a macroscopic concept. It is just a convenient construct for describing the vertical component of the net effect of all aerodynamic forces acting on a body. Sort of like "pressure" and "temperature" are convenient constructs for describing the macroscopic net effect of the motion and collisions of a whole bunch of individual molecules. In the context of lift, the top and bottom wing skins are not independent bodies. It therefore doesn't make sense to talk about the lift of the bottom skin versus the lift of the top skin, any more than it makes sense to talk about about the temperature or pressure of a single molecule. The macroscopic concept just doesn't apply at that microscopic level.

I'm sorry, I really don't know how to explain it any more clearly than the explanations already given here by myself and others. I think this discussion is going in circles at this point, so I'm out.

I think the problem with this whole discussion is that there are really two concepts at play here, but most people are only thinking about one.

The two concepts:

1. "Lift" generated by the wing.

2. Airframe forces.

As Roee stated, the concept we're referring to as lift is a macroscopic concept, and it is apparently easier to think of it as an effect on the entire wing, as if the wing were solid.

As for airframe forces, this is the concept of load applied to individual components of the aircraft, or the wing in this case. The airframe forces are something an aircraft designer would need to know to determine exactly the loads on each component that make up the aircraft. In this case, certainly the internal pressure of the wing and the differential pressure on each wing skin (top and bottom) must be taken in to account. So, for example if we assume the wings internal air pressure is approximately equivalent to static air pressure, and the pressure outside of the top skin is considerably less than static while the pressure on the outside bottom skin is only a little less than static, then the overall lift force is certainly the difference of the top outside pressure minus the bottom skin outside pressure times the wing area. However, if we need to know how many of what size rivets to hold the skins on, and how thick to make the skins and the ribs, then we are in fact interested in knowing the forces on the individual skins due to the differences in internal and external pressures and the forces they produce on these components. I think that this is the point that Paul is trying to make...

Skylor
Rambling a little late at night after a little wine...
 
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Another thing for lucubration: air has neither pressure nor temperature. It only possesses molecules that have a mean free velocity of about M1.4, V= (3*P/rho)^1/2, and what we call pressure and temperature, two sides of the same coin, are only a result of the collision of these molecules with whatever gets in their way.

aren't you leaving out gravitational forces in this little statement? no earth, no thing pulling all those molecules closer together than they would normally move.
 
WOW!, Skylor! I think you're on the right track. You also noted that I put the observation about the pressure differences and lift in the form of a query, not an outright statement.
In order to design a structure the engineer must know the loads at every point. If the engineer asks the mathematician or theoretician about the lift and he just says "Well, it's just the summation of forces acting on the whole plane.", he would be well justified in taking his .44 Magnum and using it to make a neat hole in the mathematician's forehead.
I'm really surprised at the number of respondents who skate and skirt around the whole issue with their "Lift is a result of the mass flow times the downwash velocity." and "It's the result of the summation of all of the force vectors." and such. What I've often found revealing in my discussions with others on various topics is that far too many are afraid of giving definitive answers, for fear they may look wrong in the eyes of others. This is especially true if they must put their answers down in print for all to see for all time. So what they do instead is rely on time-proven reponses to keep from showing their vulnerability, even if they are only generalizations.
I tried to give all of the theoreticians a little hint when I spoke of the structure-less construction of the Atlas missile. Along this line, what if we were to make a totally hollow wing with no internal structure; no ribs, no spar, just an applied air pressure within it. Goodyear made an inflatable plane for the military to be parachuted to a downed pilot, which he would inflate and fly off.
How in this case are the lift loads transmitted to the wing and then from it to the rest of the struture? Through air pressure? :confused:
 
skirt around the whole issue with their "Lift is a result of the mass flow times the downwash velocity." and "It's the result of the summation of all of the force vectors." and such. What I've often found revealing in my discussions with others on various topics is that far too many are afraid of giving definitive answers, for fear they may look wrong in the eyes of others. This is especially true if they must put their answers down in print for all to see for all time. So what they do instead is rely on time-proven reponses to keep from showing their vulnerability, even if they are only generalizations.

I believe I've been reading "what causes lift" theory for about 40 years now. It's always interesting, to say the least. But it's all still theory........:)

So...........keep all these ideas coming, as its fascinating in a way. If someone says their thoughts are fact..........it just isn't so. At least not yet! :D

L.Adamson ---- RV6A
 
Its a wonder anything flies at all with all the scientific knowlege we're suppressing in search of intellectual honesty.
 
I was on a jet flying a landing approach in rain, sleet, and snow (Even though sleet means a mixture of rain and snow, I threw the rain and snow in for those who have never been out of SoCal!). I was sitting ahead of the leading edge and marveled at the sight of these elements being swept up toward the leading edge as they were being illuminated in the landing light. A few days after I got home I asked an acquaintance who was an aero grad from Cal Poly SLO what caused this action. He replied that it was due to the low pressure being broadcast ahead of the wing. I asked him if that was AM or FM. When pressed, he said that was how the instructor taught the class. If only this instructor had known about the Coanda effect, he and the class would have had a much better understanding of the forces at work.
I also once acted as a TA, technical advisor, to two Cal Poly EE students on their senior projects. The one had an instructor who didn't undestand the biasing of an Op Amp in the middle of its applied voltage when operating from a single supply voltage, and the other didn't know that when you transmit 24 bits of TTL over wires that each bit wire must have its own return, or if only a single wire return was used for all 24 it would cause common-mode coupling.
The students only learn what the instructors teach them and then go on to become instructors themselves to teach some of the same falsehoods. Something along the line of educational entropy!
Try to get good propeller design in one of these aero classes. They're still discussing advance ratios and such. :eek:
 
Back after a couple days of testing the theory of differential pressure acting upon multiple airfoils in close proximity to each other (formation flying). But I didn't stay in a Holiday Inn Express, so I'm no smarter! :rolleyes:

WOW!, Skylor! I think you're on the right track. You also noted that I put the observation about the pressure differences and lift in the form of a query, not an outright statement.

Paul, I thought Skylor made some good points too, and I was going to say that I get your points (er, I mean queries ;)) about whether its the outside air's pressure on the bottom...or the outside air's pressure on the top...or the differential pressure between them...or the differential in differential pressures (outside versus inside) between the top and bottom.

Being the goof pilot I am, I'm still looking for the "Answer D: All the above" option. That's the bubble I'd fill in...but then again, since this is all theory, if you added an "E: None of the above", I'd really be in a conundrum! In some ways, we've been saying the answer is "7". You're asking if that means 0+7, 1+6, 2+5, or 3+4. I'm wondering if it matters (and yes, I live in Nevada, thus my analogy!)

However, given that a solid wing, a rotor, a prop blade and a hollow wing (or hollow wings of various structures) all generate that amorphous force we call lift when moved through a medium (yes, I know, I'm already in trouble with that one, as the wing probably doesn't generate lift, it probably facilitates its generation)...but I digress...back to the point...I think (IMHO) that truly is all the above. We each try to explain it in our own best terms. And yes, both the mathemetician and the engineer have to be able to use the info. My guess is that in this case, the engineers use a best guess model as a baseline, then they (or in some case pilots :eek:) test the model to failure, and the M's and E's scratch their heads and say, "well, that was wrong, lets try this", and we'll get Bob to test fly it. :rolleyes: My guess is they don't have the full mathematical answer, so they over-engineer it for some margin of safety. Sure if we had it perfect, we could save weight or strcuture, but as that test dummy, I like the margin.

I find it much more useful to be able to harness the resultant force than to be able to fully explain it, though it doesn't mean I (and others) won't try our best (please see my next part below)...

I'm really surprised at the number of respondents who skate and skirt around the whole issue with their "Lift is a result of the mass flow times the downwash velocity." and "It's the result of the summation of all of the force vectors." and such. What I've often found revealing in my discussions with others on various topics is that far too many are afraid of giving definitive answers, for fear they may look wrong in the eyes of others. This is especially true if they must put their answers down in print for all to see for all time. So what they do instead is rely on time-proven reponses to keep from showing their vulnerability, even if they are only generalizations.

I was on a jet flying a landing approach in rain, sleet, and snow (Even though sleet means a mixture of rain and snow, I threw the rain and snow in for those who have never been out of SoCal!). I was sitting ahead of the leading edge and marveled at the sight of these elements being swept up toward the leading edge as they were being illuminated in the landing light. A few days after I got home I asked an acquaintance who was an aero grad from Cal Poly SLO what caused this action. He replied that it was due to the low pressure being broadcast ahead of the wing. I asked him if that was AM or FM. When pressed, he said that was how the instructor taught the class. If only this instructor had known about the Coanda effect, he and the class would have had a much better understanding of the forces at work.
I also once acted as a TA, technical advisor, to two Cal Poly EE students on their senior projects. The one had an instructor who didn't undestand the biasing of an Op Amp in the middle of its applied voltage when operating from a single supply voltage, and the other didn't know that when you transmit 24 bits of TTL over wires that each bit wire must have its own return, or if only a single wire return was used for all 24 it would cause common-mode coupling.
The students only learn what the instructors teach them and then go on to become instructors themselves to teach some of the same falsehoods. Something along the line of educational entropy!
Try to get good propeller design in one of these aero classes. They're still discussing advance ratios and such. :eek:

Paul, we've met and had great conversations...I like talking with you, and wish I could figure out a way to test one of your props (still hung on the CS to FP engine/crank ramifications and how they relate to my pocketbook, but I'd still love to do it). That being said, seems to me (again IMHO) a lotta gentle (and not so gentle) bashing going on here brudda. Several great posts by guys putting it out there, and taking that risk. Several have checked out because their "time-proven responses" were seemingly dismissed (kinda felt my discussion on the canopy issue was as well, but what the hey, eh!)

You've asked some thought provoking questions...and have thrown some chum in the water to keep the discussion going...and many have responded. Some of us challenge some of the theories you (and others) postulate in response to your original query, some ask follow-on questions, and the discussion rolls on. But I need to remember that approaching and postulating responses to your queries is (and I mean this in a very complimentary way, honest) akin to trying to describe what something looks like with my eyes, to a guy that sees in the visual, and the XRAY, and the IR spectrums. :)

Guess all I'm saying is, if we are going to expand the plane (or sphere) of thought on these theoretical concepts via discussion, then perhaps attack innuendo, subtle though it may be, will turn some off. Rhetorical queries are fun, if we keep 'em fun. Nobody elected me counselor or mediator, but I like your postulations and learn a lot, so I'm just tossing a pitch for being (intellectually) nice to mere mortal pilots like me! :)

Now, do you have a theory on that initial query that we may be able to evaluate and take to that next plane (sphere)? ;)

I tried to give all of the theoreticians a little hint when I spoke of the structure-less construction of the Atlas missile. Along this line, what if we were to make a totally hollow wing with no internal structure; no ribs, no spar, just an applied air pressure within it. Goodyear made an inflatable plane for the military to be parachuted to a downed pilot, which he would inflate and fly off.
How in this case are the lift loads transmitted to the wing and then from it to the rest of the struture? Through air pressure? :confused:

OK, so we've never really resolved how the force gets applied to the wing, so I'll stick my neck out and say its transmitted via the wing structure (however it gets there, and in whatever form that wing finally takes), via wherever it interacts with the rest of the aircraft structure (wing root, etc...if this conceptual aircraft has those). Otherwise it is magic (PFM, right!). At least that's what my aircraft structures prof taught me. Now I didn't go to CP SLO, but I am a product of the California State University system (SJSU), so that may be suspect! :p (Yes, I'm kiddin' around).

So Paul, what do you think the answer to your question is?

Cheers,
Bob
 
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Fluid mechanics isn't theory, its well understood, measurable, provable, repeatable scientific fact.

The flow field over an airfoil or wing accelerates around the body as the result of a continuous distribution of pressure exerted on the fluid by the body. An equal and opposite reaction must occur on the body. This pressure distribution, acting everywhere normal to the body's surface, is what creates lift.

The integration of the pressure differential between the upper and lower surfaces, acting over the aggregate area, is the force we call LIFT, when measured normal to the free stream direction. And that is not theory.
 
That being said, seems to me (again IMHO) a lotta gentle (and not so gentle) bashing going on here brudda. Several great posts by guys putting it out there, and taking that risk. Several have checked out because their "time-proven responses" were seemingly dismissed (kinda felt my discussion on the canopy issue was as well, but what the hey, eh!)...

Guess all I'm saying is, if we are going to expand the plane (or sphere) of thought on these theoretical concepts via discussion, then perhaps attack innuendo, subtle though it may be, will turn some off. Rhetorical queries are fun, if we keep 'em fun. Nobody elected me counselor or mediator, but I like your postulations and learn a lot, so I'm just tossing a pitch for being (intellectually) nice to mere mortal pilots like me! :)

I agree with you rvmills, and I'm not as nice. There's a reason why "Stick and Rudder" by Wolfgang Langewiesche has been in print since 1944. It was written to teach regular people how airplanes fly and what they need to do to fly them. Most pilots don't need to know the kind of thick math and theory/fact presented in this thread, and get a headache thinking about it. It's great to discuss and teach constructively, but when that discussion becomes (or was created as) a vehicle for an engineer to preach to accomplished pilots and instructors about what he perceives as their stupidity, it does more harm than good.
 
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Fluid mechanics isn't theory, its well understood, measurable, provable, repeatable scientific fact.

The flow field over an airfoil or wing accelerates around the body as the result of a continuous distribution of pressure exerted on the fluid by the body. An equal and opposite reaction must occur on the body. This pressure distribution, acting everywhere normal to the body's surface, is what creates lift.

The integration of the pressure differential between the upper and lower surfaces, acting over the aggregate area, is the force we call LIFT, when measured normal to the free stream direction. And that is not theory.

Sorry, but I'm not buying it as fact. It wasn't too long ago, when it was discovered that the molecules above the wing weren't accelerating at the speed, previously thought. It used to be accepted that molecules flowing over the top, met the same set flowing across the bottom. Given, that, and plenty of other explanations, I wouldn't call the forces of "lift" as being scientific fact.

L.Adamson ---- RV6A
 
I beg to differ

Sorry, but I'm not buying it as fact. It wasn't too long ago, when it was discovered that the molecules above the wing weren't accelerating at the speed, previously thought. It used to be accepted that molecules flowing over the top, met the same set flowing across the bottom. Given, that, and plenty of other explanations, I wouldn't call the forces of "lift" as being scientific fact.

L.Adamson ---- RV6A

I'm going to beg to differ. The misconception you are referring to is known as the "Equal Transit-Time Fallacy." The fallacy was never believed by serious aerodynamicists --- it crept into the public consciousness through popularizations, notably in the Encyclopedia Brittanica. The mechanisms which produce lift have been very well understood since the early part of the 20th century, and don't involve equal transit time.

The fact is that lift can be predicted with great accuracy based on numerical codes. Those codes are based on well-understood physics. The codes can and have been verified by wind tunnel testing. By any reasonable meaning of the term, the current understanding of lift can be considered as scientific fact.

Is lift caused by pressure over the surface or momentum added to the airstream? Yes, the total lift can be found by integrating the pressure forces over the airplane, so pressure differences cause lift. But the integral momentum theorem relates the total aerodynamic force on an object (such as an airplane) to the change in momentum of the surrounding fluid*. So you can't have lift without a change in the momentum of the fluid. One can say that there is momentum in the fluid because of the lift, or one can say that there is lift because of the momentum change. Since you can't have one without the other, it's actually a bit meaningless to argue about which is more fundamental.

As to the air inside an airfoil, if its velocity is low (essentially zero with respect to the airfoil), then it has no net effect on lift. However, if the airfoil is vented, say, to the leading edge stagnation point, then the airfoil skin will balloon out on both the upper and lower surfaces, but more so on the upper surface. If the airfoil is vented to the upper surface, near the 1/4 chord point, the airfoil skin will balloon in, but more so on the lower surface. The net force on the airfoil will be the same in both cases.

----------

* Actually, the integral momentum theorem relates the lift on an aircraft to the change in the momentum in the surrounding fluid AND the pressure of the fluid in the far field. This complicates the description above in a very subtle way, especially for incompressible fluids. For incompressible fluids with fixed boundaries (as in a wind tunnel), lift on an aircraft produces no net change in the fluid momentum, but does change the pressure at the fixed boundaries.
 
'Sorry if I get what seems to be contentious in my responses; I don't mean to seem that way. I get a little perturbed when I find that someone gives me a mathematical or theoretical treatise to side-step the question at hand, so I try to show how the question is being avoided. It puts me in mind of the obfuscation that is the hallmark of a politician's reply to a question to which he doesn't want-to or can't answer. An honest response is usually something like "I've never thought about that before so I'll give it some thought and not just dismiss it off-hand." In reponding to my original query, Mel Asberry wrote that he thought that all of the lift was on the wing's bottom; at least he spoke his mind with no equivocation.

The original noise model of the tropospheric effects on a radar wave, for our guidance computer, were developed strictly by fitting a polynomial curve to observed data and had absolutely no relationship to the real world. It was good in the middle, but like so many of these attempts, went squirrely at the ends. All it took to get a true model was to look at how the radar wave propagated through the air, and model that; simple and accurate. The same wth refraction correction. Astronomers use incredibly complex parameters in a curve fit that are chosen by some hieratical formula by the astronomical priests, but a simple model, based on measured physical properties, gives outstanding results.

Now let me put forth a model. There's a plane with a hollow wing, joined only at the edges with no internal structure. The fuselage is joined to the top skin only. There's a giant man on the ground holding the plane up using both of his hands on the bottom skin, so that the weight of the plane is borne through his body to the ground. There is a tiny man inside the wing holding up the top skin, so that the weight on it passes through him to his feet on the bottom skin, and thence into the man holding it up on the outside. The little man's hands are pushing up on the top surface and his feet are pushing down equally on the bottom skin. Is he holding it up? Does the weight pass through him and the bottom skin and the giant? :confused:
 
Sorry, can't help myself again on this thread. Maybe what Larry refers to is commonly known as the boundary layer. And in fact, the molecules of air right next to the skin do not move. You can see this if you have a dusty wing - the dust (generally) will not blow off of it in flight because the dust is small enough to be within the boundary layer where no air is moving. The same thing happens in the desert and in stream bottoms - there is a boundary layer below which stuff doesn't move. Yes, there are dust storms in the desert, but these are started primarily by an external force such a someone tossing a pebble into the dirt in the wind, which has a cascading effect as more pebbles, sand, and dust are kicked up and then drop through the boundary layer to disturb other bits and pieces (which end up as the dust storm if they are tiddleywinked into the moving air above the boundary layer).

greg
 
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